Note 1: This works with the radeon driver, and with the proprietary NVIDIA driver. But, with nouveau installed, unless you use the "nomodeset" parameter in GRUB, and get a lower screen resolution because of that, you'll get an irritating red symbol, that I couldn't get rid of (https://pasteboard.co/GE6RPlp.jpg) - apparently signalling(?) that the result of the "fsck" process was OK (even if I disabled "fsck" at boot, and the output of any messages)...
Note 2: Disabling "fsck" (file system consistency check) at boot, means your computer won't be scanned for any hard drive errors when you boot. Also, disabling the output of messages at boot, means that you won't receive some of them. Therefore, use these instructions at your own risk.
So, what you have to do, is only to add the following parameters in GRUB, to your "GRUB_CMDLINE_LINUX_DEFAULT" line (by editing, as root, the file "/etc/default/grub" - and, not forgetting to run, as root, "update-grub" afterwards) so that it changes from this:
to this:GRUB_CMDLINE_LINUX_DEFAULT="quiet"
(Or, alternatively, you can substitute "fsck.mode=skip" for "fastboot".)GRUB_CMDLINE_LINUX_DEFAULT="quiet loglevel=3 fsck.mode=skip vt.global_cursor_default=0"
Explaining what the parameters do:
1) "loglevel=3" gets boot rid of any unimportant error messages (like when it complains about the absence of a proprietary firmware that you don't need).
2) "fsck.mode=skip" makes the boot process skip the "fsck" part (while "fastboot" might do something more than that(?)).
3) "vt.global_cursor_default=0" gets boot rid of the blinking cursor (but, for some reason, not at the part right before the display/login manager starts).
And, that's the best that I could find out, and do... If anyone can do better than this - like patching GRUB so that it doesn't show up at all, like in Ubuntu - please let us know how to do it. Happy hacking.